In wellbores, annular barriers are used for different purposes, such as for providing a barrier to flow between an inner tubular structure and the inner wall of the borehole. The annular barriers are mounted as part of the well tubular structure. An annular barrier has an inner wall surrounded by an annular expandable sleeve. The expandable sleeve is typically made of a metallic material, but may also be made of an elastomeric material. The sleeve is fastened at its ends to the inner wall of the annular barrier.
In order to seal off a zone between a well tubular structure and the borehole, a second annular barrier is used. The first annular barrier is expanded at one side of the zone to be sealed off and the second annular barrier is expanded at the other side of that zone. Thus, the entire zone is sealed off.
The pressure envelope of a well is governed by the burst rating of the tubular and the well hardware etc. used within the well construction. In some circumstances, the expandable sleeve of an annular barrier is expanded by increasing the pressure within the tubular structure of the well, which is the most cost-efficient way of expanding the sleeve.
When expanding the expandable sleeve of an annular barrier by pressurising the tubular structure from within, several annular barriers are expanded simultaneously. The force, i.e. pressure, required to expand the annular barriers depends on many variables, such as the size of the borehole in relation to the size of the inner tubular structure, the strength of the expansion sleeve, etc. As the size of the borehole may vary along the length of the well, the distance between the inner tubular structure and the inner wall of the borehole is not constant in the well. Consequently, different annular barriers require different pressure levels to be expanded into a contact position. However, if an annular barrier, after having been expanded into a contact position, is subject to an increasing pressure level in the well, undesirable damage of the surrounding formation or other adverse effects may be the result. An undesirable increase in the pressure in the expandable sleeve may result in a too high contact pressure between the expandable sleeve and the inner wall of the borehole, whereby the surrounding formation may crack and thereby compromise the seal effect of the annular barrier. Also, the expandable sleeve may crack or burst due to the increased pressure, thereby adversely affecting the effect of the annular barrier.